1. Field of the Invention
The present invention relates to a radiation detecting element using a tin oxide crystal and a method of manufacturing semiconductor including a tin oxide crystal having a high resistivity, which is applicable to the radiation detecting element.
2. Description of the Related Art
As a radiation detecting apparatus used to detect X-ray or γ-ray in medical or industrial application, a radiation detecting apparatus including a detecting portion using semiconductor such as Si or Ge has been known. However, in the case of using Si and Ge, a thick semiconductor layer is required, because Si or Ge has a long absorption length of the X-ray, and hence there is a constraint in downsizing the apparatus or making the apparatus thinner so that a usage range of the apparatus is limited. In Japanese Patent Application Laid-Open No. 2001-242255, a radiation detecting apparatus using CdTe, CdZnTe, HgI2, PbI2, amorphous selenium (a-Se), or the like, as a material having a relatively short absorption length of the X-ray, has been described.
Further, acquisition of an image of radiation is performed by using a sensor array including two-dimensionally arrayed radiation detecting elements. For example, an X-ray image sensor has been developed, which includes transistors and radiation detecting elements (photoconductive elements) formed on a substrate in an arrayed manner, as an image detecting apparatus for X-ray diagnosis for chest or general radiography. There is a system of converting incident X-ray absorbed in a photoconductive layer such as a-Se into charge signals by using a high electric field, accumulating the charges in a capacitor, and reading out the accumulated charges through a transistor.
In order to detect the incident X-ray completely, a sufficiently thick photoconductive layer is required to absorb the X-ray. For example, in the case of using a-Se as the photoconductive material for detecting the X-ray, the a-Se is formed as thick as about 1 mm. Further, in order to collect charges generated in the a-Se to arrive at electrodes without being trapped in a defect level in the film, a voltage as large as about 10 kV is applied between the electrodes.
However, in the materials such as a-Se, CdTe, HgI2, PbI2, and the like, which have been considered before, a highly toxic element is used.
Although there has been a report of using ZnO as a material having low toxicity and small environmental load, it has not yet led to practical use due to its low sensitivity and large dark current.